1. Field of the Invention
The present invention relates to a method of manufacturing a silicon single crystal and a silicon single crystal ingot, and more particularly, to a method of manufacturing a silicon single crystal that is grown by a Czochralski method (CZ method) and used preferably as a substrate of a semiconductor device and a silicon single crystal ingot. Further, the present invention relates to a silicon wafer, and more particularly, to a silicon wafer that is sliced from a silicon single crystal ingot grown by a Czochralski method and preferable as a substrate of a semiconductor device.
2. Description of Related Art
When a silicon single crystal is grown by the Czochralski method, kinds and distributions of defects that are included in the silicon single crystal depend on a ratio of a pulling-up velocity V of the silicon single crystal and a temperature gradient G in a growth direction in the silicon single crystal.
FIG. 16 illustrates a general relationship between V/G and kinds and distributions of defects. A pulling-up condition of FIG. 16 is set to Gc/Ge<1, when a central portion of a pulled-up single crystal is disposed in a temperature range from a melting point to 1370° C., a temperature gradient in the central portion is defined as Gc, and a temperature gradient in an outer circumferential portion is Ge.
As illustrated in FIG. 16, when V/G is large, the amount of vacancies becomes excessively large, and a minute void (defect generally called a COP: Crystal Originated Particle) that is an aggregate of the vacancies is generated. Meanwhile, when V/G is small, the amount of interstitial silicon atoms positioned between lattices becomes excessively large, and dislocation clusters that are an aggregate of the interstitial silicon atoms are generated. Accordingly, in order to manufacture a crystal that includes neither the COPs nor the dislocation clusters, V/G needs to be controlled to be in an appropriate range in a radial direction and a length direction of the crystal. First, with respect to the radial direction of the crystal, since V is constant even at any position, a structure of a high-temperature portion (hot zone) in a CZ furnace needs to be designed, such that the temperature gradient G is in a predetermined range. Next, with respect to the length direction of the crystal, since G depends on the pulling-up length of the crystal, V needs to be changed in the length direction of the crystal to keep V/G in the predetermined range. At the present time, even in a silicon single crystal whose diameter is 300 mm, a crystal that includes neither the COPs nor the dislocation clusters are massively produced by controlling V/G.
As described above, the silicon wafer that includes neither the COPs nor the dislocation clusters are massively produced by controlling V/G and used when manufacturing an electronic device. However, this wafer includes plural regions where entire surfaces are not uniform and behaviors at the time of heat treatment are different from each other. As illustrated in FIG. 16, between a region where the COPs are generated and a region where the dislocation clusters are generated, three regions including an OSF (Oxidation Induced Stacking Fault) region, a Pv region, and a Pi region exist in the order of large V/G. The OSF region includes platelet oxygen precipitates (OSF nuclei) in an as-grown state (state where any heat treatment is not performed after a crystal is grown). The OSF region is a region where OSFs are generated when being thermally oxidized at a high temperature (in a temperature range from 1000° C. to 1200° C. in general). The Pv region includes oxygen precipitation nuclei in an as-grown state. The Pv region is a region where oxygen precipitates are easily generated when heat treatments of two steps of a low temperature and a high temperature (for example, 800° C. and 1000° C.) are performed. The Pi region rarely includes oxygen precipitates in an as-grown state. The Pi region is a region where it is difficult to generate the oxygen precipitates, even though the heat treatment is performed.
Since a difference between V/G where the COPs starts to be generated and V/G where the dislocation cluster starts to be generated is very small, a pulling-up velocity V needs to be strictly managed in order to manufacture a crystal not including the COPs and the dislocation cluster. However, even though the crystal is pulled up at a targeted pulling-up velocity V, the COPs or the dislocation clusters may be generated due to various factors. This is due to the following reasons.
The CZ furnace includes some members, such as a carbon heater, a heat insulating material, and a carbon crucible. These members are continuously used while pulling-up is performed tens of times to hundreds of times. These members are temporally deteriorated and wasted due to a reaction with vapor of a silicon melt or a solution droplet, a reaction with gas generated from carbon and the silicon melt, and a reaction with a quartz crucible, and a thermal characteristic of a hot zone in the CZ furnace is temporally varied. If the temporal variation of the hot zone is generated, a temperature gradient G is varied. Even though the crystal is pulled up at the targeted pulling-up velocity V, V/G may be deviated from a designed value. Therefore, the COPs or the dislocation clusters are generated even though the crystal is pulled up at the targeted pulling-up velocity V.
Accordingly, in order to realize the targeted V/G, a profile of the pulling-up velocity V needs to be changed according to the temporal variation of the hot zone.
In the related art, a pulling-up velocity profile is set to include an OSF region. A sample that is sliced from the pulled-up crystal is decorated with Cu (copper) or subjected to heat treatment for an OSF evaluation to evaluate the width of the OSF region, and a subsequent pulling-up velocity profile is adjusted on the basis of the width (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2005-194186 and International Publication Pamphlet No. WO 99/40243). That is, if the OSF region is wide, the CZ furnace is varied in a direction where V/G increases (G decreases). In the subsequent pulling-up, the pulling-up velocity V is set to be low. In contrast, if the OSF region is narrow, the CZ furnace is varied in a direction where V/G decreases (G increases). In the subsequent pulling-up, the pulling-up velocity V is set to be high.
In these methods, since the width or the position of the OSF region is used as an index and the subsequent pulling-up velocity profile is adjusted, the OSF region is necessarily included even in a wafer that is shipped as a product. At the present time, the OSF region does not seem to affect an electronic device. However, since the OSF region is a region that includes OSF nuclei even in an as-grown state, that is, a platelet oxygen precipitate, the possibility of the OSF region becoming a factor causing a characteristic of a future electronic device to be deteriorated is high. Accordingly, in the future, necessity of a development of a method that stably pulls up a crystal not including the OSF region without using the width of the OSF region as an index of the pulling-up velocity adjustment is considered.
As the method that does not use the OSF region as the index of the pulling-up velocity adjustment, a method that estimates a vacancy concentration of crystal from the amount of decrease in an elastic constant softening of silicon due to an extremely low temperature and adjusts a subsequent pulling-up velocity profile has been suggested (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2007-261935). However, in order to realize the above method, etching is performed on the wafer sliced from the silicon single crystal to remove a processing distortion, ZnO or AlN that becomes a thin film vibrator is deposited, and an external magnetic field is applied according to necessity. In this state, an ultrasonic pulse is propagated while the wafer is cooled down in a temperature range of 25K (−248° C.) or less, a sound velocity variation of the propagated ultrasonic pulse is detected, the decrease amount of an elastic constant according to the decrease in the cooling temperature is calculated from the sound velocity variation, and a vacancy concentration in the silicon wafer is evaluated from the calculated decrease amount of the elastic constant. The processes according to the above-described sequence should be executed. Therefore, an expensive evaluation facility and a complex sequence are needed, and the above method cannot be applied to a routine inspection during the manufacturing process of the silicon single crystal.
As a method that detects crystal defects in the silicon single crystal, evaluation methods based on various principles are suggested. A generally used selective wet etching method immerses a sample in a mixed solution of a material having an oxidation action with respect to silicon and a material having an oxide dissolving action, and exposes crystal defects as unevenness (etch pit in most of cases) of the etched surface. Nitric acid or chromic acid is used as the material having the oxidation action, and hydrofluoric acid is used as the material having the oxide dissolving action. Depending on the kind of used chemical material and a mixed ratio thereof, a selected ratio of normal silicon/defect is different, and sensitivity or the kind of detectable defect is different. The selective wet etching has low sensitivity as compared with the other methods, but is simple. Therefore, the selective wet etching is still used for a crystal defect evaluation at the present time. As typical etching solutions, there are a Write solution, a Secco solution, and a Dash solution that take names of people that suggest the solutions.
An infrared tomography method that is generally used from 1990's is a method that uses a difference in refractive indexes of silicon and a defect. Since infrared rays transmit the silicon, a defect in the wafer can be evaluated. This method has high sensitivity to oxygen precipitates or COPs, as compared with the selective wet etching.
In Japanese Patent Application Laid-Open (JP-A) Nos. 2000-58509 and 2007-123542, a defect detecting method using reactive ion etching (RIE) will be described. This method exposes an oxygen precipitate, such as a BMD, by heat treatment, and performs the RIE to a sample under the condition where a selected ratio of Si/SiO2 is high. Thereby, the oxygen precipitate (SiO2) is not etched and exposed as a protrusion. If the condition where a selected ratio of Si/SiO2 is high is selected, it is reported that a defect evaluation having high sensitivity is possible as compared with the infrared tomography method.
It has been so far strongly required to provide a wafer where an oxygen precipitate is formed with a high density and a gettering capability is excellent. However, if the oxygen precipitate is one kind of crystal defect and exists on a surface layer of the wafer where a device is formed, this causes a device defect. Therefore, an annealed wafer where high-temperature heat treatment is performed on a silicon wafer having an oxygen precipitate, and the oxygen precipitate existing on the surface layer of the wafer where the device is formed is removed, or an epitaxial silicon wafer where an epitaxial film is formed on a surface of a wafer having an oxygen precipitate has been developed. However, any of new processes need to be additionally executed with respect to the wafers, productivity is decreased, and a manufacturing cost is increased.
In recent years, an insulated gate bipolar transistor (IGBT) has been developed. Like an LSI, such as a memory, the IGBT is not a device that uses only the neighboring portion of the surface of the wafer in a horizontal direction but a device that uses the wafer in a vertical direction (wafer thickness direction), and a characteristic thereof is affected by a quality of the bulk of the wafer. Therefore, the oxygen precipitate in the wafer as well as the oxygen precipitate in the wafer surface layer needs to be reduced. In recent years, a wafer that is not limited to the IGBT wafer, greatly reduces the risk of an impurity contamination due to a cleaned device, does not depend on the gettering ability as a quality required in the wafer, and a wafer that reduces, without limitation, not only the COPs and the dislocation clusters but also the oxygen precipitate as one kind of crystal defect is anticipated as a next-generation wafer to be requested.
In general, in order to decrease the oxygen precipitate in the wafer, the oxygen concentration of the crystal can be decreased. At a current situation, a low-oxygen silicon single crystal ingot where an oxygen concentration is decreased to 3×1017 atoms/cm3 by adjusting a crucible rotation velocity or a crystal rotation velocity using a magnetic-field-applied Czochralski method (MCZ method) for applying a magnetic field can be manufactured (oxygen concentration described in this specification is a value that is measured by Fourier transformation infrared spectrophotometry standardized in ASTM F-121 (1979)).
However, when silicon single crystal that includes a non-defect region where an oxygen precipitate is small and COPs and dislocation clusters do not exist is grown, a crystal region (Pv region) where an oxygen precipitate is active needs to be excluded as maximal as possible. However, the oxygen precipitation in the Pv region may be decreased due to the decrease in the oxygen concentration in the crystal, and the difference of the oxygen precipitate distributions in the Pv region and the Pi region may be extremely reduced. Therefore, in the defect distribution evaluation by the currently executed oxygen precipitate evaluation heat treatment (two-step heat treatment at a high temperature and a low temperature), it becomes difficult to determine a boundary between the Pv region and the Pi region.
In the oxygen precipitate evaluation heat treatment, two-step heat treatment of low-temperature heat treatment (at the temperature of 800° C. for 4 hours) and high-temperature heat treatment (at the temperature of 1000° C. for 16 hours) is performed in an oxygen atmosphere, an oxygen precipitation nucleus in the crystal is grown by the heat treatments and exposed as the oxygen precipitate, and a density distribution is evaluated by an optical microscope. However, in the evaluation method, the minute oxygen precipitation nucleus cannot be exposed, and a density or size of the exposed oxygen precipitate depends on the oxygen concentration. Additionally, due to high-temperature heat treatment for a long time, an oxygen precipitation nucleus having a small size in the crystal may be removed. Accordingly, in the defect distribution evaluation by the oxygen precipitation evaluation heat treatment, the risk of the minute oxygen precipitate existing in the crystal is high, and it is difficult to grow a silicon single crystal from which the crystal region where the minute oxygen precipitate exists is excluded.
Meanwhile, a copper decoration method that contaminates a surface of an evaluation sample with copper, is subjected to heat treatment at a temperature in a temperature range of 800° C. to 1000° C. for 3 to 20 hours to diffuse the copper into a sample, and exposes a defect of the crystal surface is effective because the crystal defect can be detected with high sensitivity. However, since the high-temperature heat treatment needs to be performed, the minute oxygen precipitation nucleus may be removed, similar to the oxygen precipitation evaluation heat treatment. Furthermore, since the plural heat treatments need to be performed for a long time, a large amount of time may be needed for the evaluation, and a time lag until the evaluation result is fed back to the single crystal growing condition may increase.
Meanwhile, according to the RIE method that is described in JP-A No. 2000-58509, the minute oxygen precipitate can be detected. In JP-A No. 2000-58509, it is only reported that the oxygen precipitate such as the BMD exposed by the heat treatment can be evaluated, and an evaluation with respect to a silicon wafer in an as-grown state is not described. At the technology level of when JP-A No. 2000-58509 is filed, only OSF nuclei become a problem as a defect included in the silicon wafer in an as-grown state. Since the OSF nuclei are easily exposed by the thermal oxidation, the OSF nuclei do not need to be detected in the as-grown state. Further, at the technology level of when JP-A No. 2000-58509 is filed, it cannot be determined whether or not the Pv region includes an oxygen precipitation nucleus in the as-grown state.
JP-A No. 2007-123542 describes that the OSF nuclei can be exposed by the RIE method. However, like the oxygen precipitate that is included in the Pv region, whether or not the RIE method is effective with respect to the oxygen precipitate more minute than the OSF nuclei are not described. This is because the oxygen precipitate included in the Pv region is generally needed as a getter site, and does not need to be removed.